Since DNA is the primary target for radiation-induced lethality, it is likely that differences in cellular radiosensitivity will involve differences in mechanisms of induction of DNA damage or the repair of that damage. Of particular interest is damage that may be repairable only under certain suboptimal conditions of growth. Such 'potentially lethal damage' (PLD) is very relevant to the tumor situation, where a large number of cells are not dividing because of suboptimal growth conditions within the tumor. These cells are probably radioresistant due to PLD repair and may later be recruited into the actively dividing fraction of calls. Virtually nothing is known about the underlying biochemistry of PLD repair; however, DNA repair has been implicated. This is consistent with the fact that many inhibitors of PLD repair are also inhibitors of cellular enzymes thought to be involved in DNA repair. In research proposed here, we will study the effects of PLD repair inhibitors on two different DNA repair processes -- strand-break and base-damage repair. We will assess the rates and overall extent of each type of repair as well as the biochemistry of the affected repair. Knowledge of the mechanisms of these processes may allow for rational design of pharmacological DNA repair inhibitors to enhance and augment tumor radiotherapy. We will also study DNA repair enzymes in tumors to determine whether radiation resistance can be explained in terms of intrinsic DNA repair capabilities. Work in our laboratory has shown that Ewing's sarcoma cells have high levels of poly(ADP-ribose) polymerase and that these cells are radiosensitized by 3-aminobenzamide, a potent inhibitor of the enzyme. This suggests that poly(ADP-ribose) polymerase may play a role in protecting cells from radiation. We will investigate the biochemistry of poly(ADP-ribose) polymerase in Ewing's sarcoma cells and assess the involvement of the enzyme in DNA repair. This will serve as a model system from which we will expand to study other DNA repair enzymes in tumors. Lastly, we will investigate the mechanisms of resistance and sensitization, witnessed in other projects of the program, by studying effects on the DNA. This will include radiation resistant and sensitive tumors (Project 1), oncogene transfected cells (Project 2), and oligonucleotide treated cells (Project 4).